All of GWR's Class 150/2s are being refurbished at Wolverton (Porterbrook units) or Wabtec Doncaster (Angel Trains units).
GWR's 158s are being refurbished at Bristol Barton Hill.
GWR's Class 165s and 166s are being repainted at Wolverton, with additional mods for air conditioning being undertaken at Long Marston.
GWR's mini-HSTs are being modified & overhauled at Wabtec Doncaster, with Night Riviera Sleeper coaches being modded/overhauled at Wolverton.
SWR's refurbished Class 442s are being refurbished at Eastleigh, while their Desiro fleet is having their refurbishment at Northam (their main depot).
SWR's 158/159 fleet are being overhauled at Brush (Loughboro'); more recent examples have returned in the new SWR livery. No interior changes yet.
ScotRail's HSTs (former GWR examples) are split between L'boro' & Doncaster; powercars at Brush, coaches at Donny.
Their 158s are being done at Springburn, and their 156s at Kilmarnock (I may be wrong on that one).
Northern's fleet is quite dispersed; their 319s are being overhauled/repainted into the new franchise colours at Wolverton, but the 769 project is taking place at Lougborough.
Their 150s & 155s are being refurbished at Wabtec Kilmarnock (along with their 3-car 158s); their 2-car 158s and their newly acquired 170s are being overhauled at Crewe LNWR.
Greater Anglia's 170 fleet is being overhauled at Crewe LNWR.
Their 321 "Renatus" programme is taking place at Wabtec Doncaster.
Southeastern's Class 375 fleet is nearing the end of the refurbishment programme at Derby Litchurch Lane; no word yet on whether the 376s will follow suit.
Their 465 fleet is currently having PRM-compatible toilets installed at Doncaster.
I think TPE's Class 185 refurb programme may be done in-house at Ardwick (though I could be wrong).
Virgin's Pendolino fleet is currently being refurbished at Alstom's new Widnes facility.
And finally, West Midlands Trains:
Their Class 350 fleet (in the London Northwestern pool) is due to be refurbished at Bletchley at some point next year. No word on their other fleets though.